High dry strength-low wet strength paper



l id United States Patent 0 HIGH DRY STRENGTH-LOW WET STRENGTH PAPER John J. Padbury, Oltl Greenwich, and Walter M. Thomas, Darien, Conn., and Walter H. Schuller, Kendall, Fla., assignors to American Cyanamid Company, New York, N.Y., a corporation of Maine No Drawing. Filed Dec. 6, 1957, Ser. No. 700,992

6 Claims. (Cl. 162-168) The present invention relates to paper of substantially improved dry strength but substantially unimproved wet strength and includes the treated cellulose fibers themselves, with and without applied sizing material, and similar water-laid fibrous cellulosic webs prepared therefrom, such as cardboard, hardboard, shaped pulp articles, and the like. The invention includes methods for manufacturing such paper by the addition of the copolymer hereinafter defined to beater pulp. 7

It is known that paper of improved dry strength results when appropriate ionic resins are added to beater pulp so that the fibers in the ultimate dry paper are bonded together by an adsorbed or precipitated content of the resin. However, such resins generaly impart sub- 7 stantial wet strength as well. Wet strength is unnecessary in the use of letter paper, book paper, and newsprint, and in such papers is undesirable because it renders the pulping of broke and scrap more difficult.

The discovery has now been made that paper of sub H stantially improved dry strength but of substantially unimproved wet strength results when the fibers in paper are bonded together by a small uniformly adsorbed amount of a normally water-soluble cationic linear acrylamide-vinylpyridine copolymer, provided the weight of combined acrylamide in the copolymer is at least 75% of the total weight thereof and provided the molecular weight is at least 10,000. We have found that paper containing to 5% of such copolymer has a dry tensile strength up to about 20% higher than would otherwise be the case, While still having negligible wet strength. This result was surprising because amide groups are nonionic, and it was not known that polymers containing only tertiary amine groups as substantially the only ionic groups would prove useful for the purpose.

The paper of' the present invention, broadly, is prepared by forming an aqueous suspension of papermaking cellulosic fibers, adding thereto sufiicien't of an aqueous solution of acrylamide-vinylpyridine copolymer as more particularly described below so that bteween 4% and 5% thereof are adsorbed by said fibers, the weight of copolymer being based on the dry weight of the fibers, sheeting the fibers to form a cellulosic web, and heating the web until dry at a temperature between 190 F. and 250 F. to develop the strengthening properties of the copolymer thereon.

The drying step referred to forms a polymer bond be-, tween the fibers. The copolymer referred to is thermoplastic but evidently combines with the cellulose molecule in some way as dry strength values are generally higher when the web is dried at elevated temperatures than when it is dried at room temperature.

It has long been known that the addition of rosin size and alum to beater pulp paper results in the development of paper of very considerably decreased dry strength. Typically the addition of 1.5% of rosin, size (based on the dry weight of the fibers) together with sufiicient alum to cause precipitation causes a decrease of 5 %to 10% in the dry strength of the paper. It is an important feature of the invention that this detrimental weakening action can be very largely counteracted by the addition of polymer without more than insignificant detriment to the sizing. As a result, it now becomes possible to manufacture rosin sized paper having substantially the high dry strength and negligible wet strength of paper which has not been sized at all.

The copolymer of the present invention is added to the cellulose pulp in amounts sufficient to deposit between about 0.1% and 5% of the copolymer on the pulp. Adsorption of the copolymer is rapid and largely complete; the amount present in any one instance can be determined by nitrogen analysis of the dry paper. The economic range extends between about A% and 3%, as in this range the amount of dry strength imparted is greatest per increment of polymer added. The wet strength of paper thus obtained, at ordinary book paper basis weight, is

generally less than 2 lb. per linear inch, a negligible value. Such paper can be repulped in ordinary beater equipment and does not require treatment with steam jets or chemicals as respectively shown in U.S. Patent No. 2,423,097'and House et al. copending application Serial No. 533,259, filed on September 8, 1955, now U.S.

Patent No. 2,872,313.

The copolymers of the present invention may be applied at any customary point in the papermaking system ahead of the wire. To facilitate metering, the copolymer is preferably added as an aqueous solution of 5%10% strength. Alum, at least in the range of a few percent based on the weight of the fibers, is not detrimental and generally appears to assist adsorption of the polymer.

I The pH of the pulp suspension at the point of addition of the copolymer should be acid, and values between about 4 and 6 are preferred. After adsorption of the copolymer is complete, the pH of the pulp may be raised to neutrality and even up to pH 9 if desired, permitting production of alkaline paper containing acid-sensitive filling material such as calcium carbonate. Optimum strength is generally developed when the fibers are sheeted at a pH between about 4 and 7.5.

The copolymer used in practicing the present invention is normally water-soluble and has a molecular weight in excess of 10,000. We have found that copolymers having a molecular weight in excess of about 100,000 yield better dry strength and accordingly are preferred. On the other hand, polymers having a molecular weight in excess of about 1,000,000 are so viscous as to be unduly difficult to pump and are only slightly better than lower weight polymers. Accordingly, we prefer to employ polymers having molecular weights in the range of 100,000 1,000,000.

. The copolymer is prepared by any convenient method,

and we prefer to synthesize it by direct polymerization of a water-soluble vinylpyridine with a water-soluble acrylamide in the presence of a persulfate catalyst. The

vinyl acetate (with or without subsequent hydrolysis),

acrylamide, methyl acrylate, styrene, and vinyl chloride. Such additional components are non-ionic and have negligible strengthening effect so that they act as diluents. The copolymer employed in the present invention is bydrophiiic, and water-soluble, linear, and cationic and is composed of substantially straight carbon chains having carbamyl and pyridyl groups attached thereto, together, if desired, with alkyl, aryl, nitrile, hydroxy, carbalkoxy, ether, and ketone groups as diluents. The copolymer in the paper of the present invention is not thermosetting but on the contrary is thermoplastic.

The invention will be further described by the following examples. These examples represent further embodiments of the invention and are not to be construed as limitations thereon.

The copolymer contains 75%99% by weight of the acrylamide and 25 %1% by weight of the vinylpyridine. The amount of e o-reacted diluent material is sufficiently low not to require variation in this ratio. The invention thus includes paper containing a copolymer made from 85% acrylamide, 10% vinylpyridine, and vinyl acetate.

Example I The following illustrates the preparation of an acryl- 2 placed in a round-bottom flask equipped as in Example 1 above. The temperature of the reaction mixture was raised to 60 C. by means of the water bath in which the flask was immersed. T o the solution was added 0.10 gm. of ammonium persulfate and the mixture stirred for two and one-half hours, temperatures between 60 C.88 C. The product was diluted to solids with water. The Brookfield viscosity of the diluted solution was 25 cps. at 25 C. The solution exhibited a pH of 8.

Example 3 The copolymers of Examples 1 and 2 were tested according to standard laboratory method wherein a pulp is prepared of beaten papermaking fibers at a consistency of 0.6% and a-Green freeness of 500 cc., the pH adjusted to 4.5 by addition of hydrochloric acid, aliquots withdrawn, copolymer added, the pH readjusted, the suspensions gently stirred for five minutes, the pulp sheeted on a Nash handsheet machine and the sheets dried for two minutes at 240 F. on a laboratory drum drier. The sheets were adjusted at 50% relative humidity and 73 F. for 24 hours and their dry and wet tensile strengths determined.

The copolymers were added as 5% aqueous solutions; the fiber used, the composition of the copolymer and the amount added are shown in the table below.

Results are as follows:

copolymer Tensile Strength,

' Lb./in.

Run No. Fiber Used Compn. She Dry PerpH Ex. cent Wet Per- Per- Added Per- Found 4 cent cent Found 4 cent AM I MVP 1 Inc.

1 Bleached North. Kr f 4. 5 30. 3 0. 6 2 dn 95' 5 1 3. 0 4. 5 33. 8 11 5 0. 6 3 do 95 5 2 3.0 4.5 34.4 13 5 0.8 4 Unbl. North. Kraft 2 4. 5 31.9 0. 6 5 do 85 15 2 3. 0 7. 0 37. 0 16.0 0. 9

1 Acrylamlde. 9 2-Methyl-5-vinylpyridlne. 8 Example 4 Adjusted to 50 lb. basis weight x 40"]500 ream).

present invention, wherein the acrylamide is 85% of the weight of the copolymer and the copolymer has a molecular weight in excess of 100,000.

To gm. of water was added 17 gm. of acrylamide, 3.0 gm. of freshly-distilled 2-methyl-5-vinylpyridine, and 14.0 ml. of 2 N sulfuric acid. The resultant solution, of 151-13, was placed in a round-bottom flask equipped with a thermometer, stirrer, reflux condenser, gas inlet tube, and two dropping funnels. The system was swept with nitrogen through the gas inlet tube and a stream of nitrogen passed through the system during the entire polymerization. A solution of 0.10 gm. of ammonium persulfate in 15 gm. of water was placed in one dropping funnel and a solution of 0.05 gm. of potassium metabisulfite in 15 gm. of water placed in the other. The catalyst solutions were added dropwise over a twenty-minute period at equal rates. The reaction mixture was held between 41 C. and 43 C. by the use of a thermostatted bath. Stirring was continued for four and one-half hours, reaction temperature 43 C., following the addition of the catalyst. The reaction mixture was diluted to 10% solids. This diluted sample exhibited a Brookfield viscosity of 65,000 cps. at 25 C. The pH was 2.5.

Example 2 The following illustrates the preparation of an acrylamide-vinylpyridine copolymersimilar to that of Example 1 wherein the copolymer has a molecular weight between 10,000 and 100,000.

A solution of 17 gm. of acrylamide and 3 gm. of freshly distilled 2-methyl-5-vinylpyridine in gm. of water was We claim:

1. Paper of substantially improved dry strength but of substantially unimproved wet strength composed of a water-laid web of cellulosic papermaking fibers bonded together by 0.1% to 5%, based on the dry weight thereof, of a normally water-soluble cationic linear acrylamidevinylpyrl dine copolymer uniformly adsorbed thereon, the combined acrylamide and vinylpyridine in said copolymer being respectively 75 %99% and 25%1% of the weight thereof, said polymer having a molecular weight in excess of 10,000.

2. Paper according to claim 1 wherein the combined acrylamide in said copolymer is between and of the weight thereof.

3. Paper according to claim 1 having a neutral pH.

4. Paper of substantially improved dry strength but of substantially unimproved wet strength composed of a water-laid web of cellulosic papermaking fibers bonded together by /2 to 3%, based on the dry weight thereof, of a normally water-soluble cationic linear acrylamide-2- methyl-5-vinylpyridine copolymer uniformly adsorbed thereon, the combined acrylamide in said copolymer being between 85% and 95% by weight thereof, said polymer having a molecular weight in excess of 100,000.

5. Process for the manufacture of paper of substantially improved dry strength but of substantially unimproved wet strength, which comprises forming an aqueous suspension of papermaking cellulosic fibers, adding thereto sufiicient of an aqueous solution of a cationic linear acrylamide-vinylpyridine copolymer having a molecular weight in excess of 10,000, the combined acrylamide and vinylpyridine in said copolymer being respectively 75 99% and 25% 1% of the weight of said copolymer, to adsorb on said fibers between 0.1% and 5% of said copolymer based on the dry weight thereof, sheeting said fibers to form a cellulosic web and heating said web until dry at a temperature between 190 F. and 250 F. to develop the strengthing properties of the copolymer thereon.

6. Process according to claim 5 wherein the pH of the suspension upon addition of the copolymer is 4 to 6, and the suspension is sheeted at a neutral pH.

References Cited in the tile of this patent UNITED STATES PATENTS Fowler et a1. May 20, 1958 

5. PROCESS FOR THE MANUFACTURE OF PAPER OF SUBSTANTIALLY IMPROVED DRY STRENGTH BUT OF SUBSTANTIALLY UNIMPROVED WET STRENGTH, WHICH COMPRISES FORMING AN AQUEOUS SUSPENSION OF PAPERMAKING CELLULOSIC FIBERS, ADDING THERETO SUFFICIENT OF AN AQUEOUS SOLUTION OF A CATIONIC LINEAR ACRYLAMIDE-VINYLPYRIDINE COPOLYMER HAVING A MOLECULAR WEIGHT IN EXCESS OF 10,000, THE COMBINED ACRYLAMIDE AND VINYLPYRIDINE IN SAID COPOLYMER BEING RESPECTIVELY 75%99% AND 25%-1% OF THE WEIGTH OF SAID COPOLYMER, TO ADSORB ON SAID FIBERS BETWEEN 0.1% AND 5% OF SAID COPOLYMER BASED ON THE DRY WEIGHT THEREOF, SHEETING SAID FIBERS TO FORM A CELLULOSIC WEB AND HEATING SAID WEB UNTIL DRY AT A TEMPERATURE BETWEEN 190*F. AND 250*F. TO DEVELOP THE STRENGTHING PROPERTIES OF THE COPOLYMER THEREON. 